BIOS memory address decoder for providing an extended BIOS memory address space by reclaiming a portion of non-BIOS address space

Abstract

A BIOS address decoder for addressing an extended BIOS memory for storing additional microprograms in a computer system. A system component is connected from the bus for receiving program instruction addresses in a first address range and providing corresponding BIOS memory addresses in a corresponding first BIOS address range. The BIOS address decoder includes the system component to receive a first subset of bus address bits representing bus addresses in the first instruction address range and responsive to the first subset of bus address bits to generate corresponding BIOS addresses in the first BIOS address range and a BIOS address indication indicating that the first subset of bus address bits indicates a bus address in the first instruction address range. An extended BIOS decoder is connected from the bus to receive a second subset of bus address bits representing bus addresses including an extended instruction address range and from the system component to receive the BIOS address indication output and is responsive to the second subset of bus address bit and the BIOS address indication for generating an extended BIOS memory enable output having a first state when the second subset of bus address bits does not represent a bus address including the extended instruction address range and a second state when the second subset of bus address bits represent a bus address including the extended instruction address range.

What is claimed is: 
     
       1. In a computer system including a processor for performing operations on data, a memory for storing programs including sequences of program instructions for controlling the operations of the processor, a bus interconnecting the processor and memory for the transfer of program instructions, data and addresses therebetween, a BIOS memory for storing BIOS microprograms for controlling operations of the processor wherein the BIOS microprograms correspond to program instructions and include sequences of microinstructions for directing corresponding operations of the processor, and a system component connected from the bus for receiving program instructions as bus addresses in a first instruction address range and providing corresponding BIOS memory addresses in a corresponding first BIOS address range normally reserved for BIOS microprograms to the BIOS memory for reading corresponding BIOS microprograms from the BIOS memory to control the operations of the microprocessor, a BIOS address decoder for addressing an extended BIOS memory for storing additional microprograms, comprising: the system component connected from the bus to receive a first subset of bus address bits representing bus addresses in the first BIOS instruction address range normally reserved for BIOS microprograms and responsive to the first subset of bus address bits to generate corresponding BIOS addresses in the first BIOS address range normally reserved for BIOS microprograms and a BIOS address indication indicating that the first subset of bus address bits indicates a bus address in the first instruction address range normally reserved for BIOS microprograms, and   an extended BIOS decoder connected from the bus to receive a second subset of bus address bits representing bus addresses including an extended BIOS instruction address range outside of the first instruction address range normally reserved for BIOS microprograms and from the system component to receive the BIOS address indication output and responsive to the second subset of bus address bits and the BIOS address indication for generating an extended BIOS memory enable output having a first state when the second subset of bus address bits does not represent a bus address including the extended BIOS instruction address range outside of the first instruction address range normally reserved for BIOS microprograms and a second state when the second subset of bus address bits represent a bus address including the extended BIOS instruction address range outside of the first instruction address range normally reserved for BIOS microprograms,   wherein the BIOS addresses generated by the system component and the extended BIOS memory enable output generated by the extended BIOS decoder represent BIOS addresses in an extended BIOS address range that concurrently includes both the first BIOS address range residing in the BIOS memory and normally reserved for BIOS microprograms and the extended BIOS instruction address range residing in the extended BIOS memory outside of the first instruction address range normally reserved for BIOS microprograms.   
     
     
       2. The extended BIOS address decoder of claim 1, further comprising: a BIOS memory connected from the system component to receive the BIOS addresses in the first BIOS address range and from the extended BIOS decoder to receive the extended BIOS memory enable output and responsive to the BIOS addresses and to the extended BIOS memory enable output in the first state to provide microprograms from a first set of microprograms, and   an extended BIOS memory connected from the system component to receive the BIOS addresses in the first BIOS address range and from the extended BIOS decoder to receive the extended BIOS memory enable output and responsive to the BIOS addresses and to the extended BIOS memory enable output in the second state to provide microprograms from an extended set of microprograms.